Polypropylene is a synthetic resin which has high rigidity and mechanical strength but of which the rate of crystallization is relatively slow. In some applications, therefore, its crystallinity after molding is low and its rigidity is reduced accordingly because of the formation of relatively large spherulites, molded articles of the polymer have inferior transparency and may have a debased commercial value.
Some attempts have been made heretofore to improve the crystallinity and transparency of polypropylene. For example, it is known that when an aluminum or sodium salt of an aromatic carboxylic acid (Japanese Laid-Open Patent Publication No. 80329/1983) or an aromatic carboxylic acid, an aromatic metal phosphate or a sorbitol derivative (Japanese Patent Publication No. 12460/1980) is added, it becomes an agent for forming a crystal nucleus (to be referred to as a nucleus forming agent), and reduces the aforesaid problem. Among these nucleus forming agents, the sorbitol derivative exhibits a particularly good nucleus forming effect, but is limited in its use because it bleeds out from the resin to contaminate rolls during film formation and give an offensive odor during processing. The aluminum salt of an aromatic carboxylic acid which is frequently used in general acts as a nucleus forming agent, but has a very small effect in improving the transparency of polypropylene. Furthermore, a film formed from polypropylene containing this aluminum salt develops numerous voids.
Japanese Patent publication No. 32340/1970 describes that to improve the transparency of polypropylene, a three-component copolymer is prepared by compolymerizing propylene, an .alpha.-olefin having 4 to 18 carbon atoms and 3-methylbutene-1. The transparency of this copolymer is still unsatisfactory.